Generally, a rain gauge is used to measure rainfall or snowfall as constituent weather conditions representing the state of the atmosphere. A conventional example of the rain gauge is configured such that the depth of rainwater collected in a cylindrical vessel having a diameter of 20 cm is expressed in millimeters.
There are various kinds of rain gauges including a cylindrical storage rain gauge, self-registering storage rain gauge, tipping-bucket rain gauge, weighing rain gauge, load-cell rain gauge, etc. These rain gauges are classified by whether they utilize a weight-measuring method or a volume-measuring method. Of the above mentioned rain gauges, the cylindrical storage rain gauge and the self-registering storage rain gauge utilize a volume-measuring method, and the tipping-bucket rain gauge, the weighing rain gauge, and the load-cell rain gauge utilize a weight-measuring method.
According to a recommendation of the World Meteorological Organization (WMO), with respect to data measured by a current universally-used Automated Surface Observing System (ASOS), the minimum measuring unit of rainfall is 0.2 mm, and more preferably, is 0.1 mm. On the basis of the recommendation of the WMO, for example, tipping-bucket rain gauges, having a resolution on the level of 0.1 mm or 0.5 mm, have been used most frequently in the measurement of rainfall using the Automated Surface Observing System installed/managed by the Korea Meteorological Administration. However, other tipping-bucket rain gauges, having a resolution on the level of 0.2 mm or 1.0 mm, are also used in associated organizations.
In Korea, two tipping-bucket rain gauges, which have a resolution on the level of 0.1 mm and 0.5 mm, respectively, have been installed and managed in meteorological observation stations. Specifically, since Jan. 23, 2002, a tipping-bucket rain gauge on the level of 0.1 mm has been used when the accumulated amount of rainfall is less than 0.5 mm, and a tipping-bucket rain gauge on the level of 0.5 mm has been used when the accumulated amount of rainfall is more than 0.5 mm. However, this is far from satisfactory in consideration of the recommendation of the WMO.
Another example of rain gauges utilizing the weight-measuring method includes a 200 mm-capacity weighing rain gauge using a load-cell. The 200 mm-capacity weighing rain gauge, however, has a problem in that it should be forcibly drained if the amount of rainwater collected in the rain gauge reaches the maximum capacity of the rain gauge. Another problem of the weighing rain gauge is that it may have a measurement error when the load-cell is affected by wind load on a windy day.
Meanwhile, it has been found that the Automated Surface Observing system has degradation in the accuracy of values measured by the tipping-bucket rain gauge as the intensity of rainfall gradually increases. To solve this problem, recently, weighing rain gauges have been mainly used in America and Europe. As reported in results of a field test by the Royal Netherlands Meteorological Institute (KNMI), weighing rain gauges have several advantages in that they can rapidly sense solid rainfall and are less sensitive to impurities and thus, can reduce an error in the measurement of rainfall due to the interference of insects, leaves of trees, dusts, excrement of birds, etc., and also, the weighing rain gauges use a more simplified measuring method and have an ease in the removal and repair/maintenance thereof as compared to electronic rain gauges. However, it was also reported that the weighing rain gauges still suffer from a measurement error due to wind, and are inappropriate to measure rainfall in the open air.
Here, considering the configuration of a conventional tipping-bucket rain gauge currently used in the Meteorological Administration, it uses a weight-measuring method in which rainwater entering a hole at the top of the rain gauge is collected in two triangular buckets by passing through tubes and the two buckets are alternately tipped like a seesaw to operate a lead switch for recording the amount of rainfall. Once one of the buckets is filled with a predetermined amount of rainwater, the bucket is tipped downward to drain the rainwater filled in the bucket. Then, the other bucket will be filled with rainwater. With this design, since it takes a time of about 0.3 second to alternate one bucket with the other bucket, there is a problem in that rainwater is directly drained during the time of about 0.3 second without being filled in any one of the buckets, thereby causing an error in the measurement of rainfall. Accordingly, the tipping-bucket rain gauge has a problem in that it measures a smaller amount of rainfall than the actual amount of rainfall. In operation of a conventional example of the tipping-bucket rain gauge, once rainwater enters a discharger through a rainwater reservoir, the rainwater is collected in triangular buckets (i.e. water sumps) through drain tubes. If the level of the collected rainwater in one of the buckets reaches 0.5 mm or 0.1 mm, the bucket is tipped by the weight of the collected rainwater. There are provided a pair of buckets, i.e. two buckets such that the two buckets are alternately tipped to operate the lead switch (or mercury) for generating a pulse signal.
Summarizing good and bad points of the tipping-bucket rain gauge depending on an observation resolution thereof, a rain gauge having a resolution on the level of 0.1 mm is efficient to precisely measure the amount of rainfall up to 0.1 mm when the amount and intensity of rainfall is small and low. On the other hand, a rain gauge having a resolution on the level of 0.5 mm has a function of measuring rainfall only at the unit of 0.5 mm and cannot measure a medium-level unit of, for example, 0.1˜0.4 mm, 0.6˜0.9 mm, etc. Furthermore, when the intensity of rainfall increases upon a heavy rain, the tipping-bucket rain gauge has a shortened tipping period, and suffers from an excessive measurement error.
Meanwhile, in the case of a conventional self-registering storage rain gauge (of a siphon type), once rainwater is introduced into a storage tank through a water reservoir, a float received in the tank is raised according to the level of rainwater, thereby allowing the amount of rainwater to be recorded by a pen connected to a shaft of the float. Then, if the level of rainwater reaches 20 mm, the tank is automatically opened through a siphon such that the level of rainwater in the tank is lowered down to a graduation of zero.
In use of the conventional self-registering storage rain gauge, it should be noted that the storage rain gauge should be previously subjected to a siphon test before it rains because a basic amount of rainwater remained in the storage tank will be evaporated after a long spell of dry weather. Further, since it takes a time of about 16 seconds to drain a rainfall of 20 mm through the siphon, it is impossible to measure rainfall during 16 seconds. Another problem of the self-registering storage rain gauge is that a predetermined amount of rainwater always stored in the storage tank is frozen in winter and cannot be used.
As will be understood from the above description, there is a need to develop a rain gauge capable of solving the above described problems of the conventional tipping-bucket rain gauge and storage rain gauge and taking only advantages of these gauges, in order to achieve a more accurate measurement of rainfall.